Is NFPA 20 Suction the Minimum for Service Size?

9/6/2021

I am looking at a project that shows 6-inch underground lead-in to a riser room. The specs however call out the install of a 1,000 gpm pump. NFPA 20 shows that for a 1,000 gpm pump suction sizing should be no less than 8-inch diameter.

Is a 6-inch even allowed for this application?

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6 Comments

Jay Richardson

9/6/2021 08:13:25 am

The minimum suction size of 8in is required only for 10 pipe diameters (80 inches) upstream of the pump suction flange, to reduce the water velocity just prior to entering the pump. Yes, your underground is permitted to be 6in supplying a 1000 gpm fire pump, up to 80 in before the suction flange.

Glenn Berger

9/6/2021 08:14:18 am

The 6 inch pipe will probably be too small for a 1000 gpm pipe. With a 1,000 you will need to test the pump at 1500 gpm (150% of pump rating, there are limited exceptions to this rule).

Prior to proceeding, have a fire hydrant flow test perform and check to see what is the flow and pressure at the pump suction.

JON DIMANLIG

9/6/2021 08:20:56 am

Per NFPA 24, Pipe smaller than 6 in. (152.4 mm) in diameter shall not be installed as a private service main supplying hydrants. So you may increase you pipe diameter based on the pump flow capacity

Jesse

9/7/2021 07:26:53 am

I agree with Jay. But my concern is more regarding the 150% flow and the ability of the 6" to supply it.

Alex Riley

9/7/2021 08:48:17 am

HI Anonymous,

A little late here. I believe Jay hit it on the money, but I wanted to provide the code sections.

As you mentioned, NFPA 20 (2019) Table 4.28 states a pump with a rating of 1000 GPM, the minimum suction size is 8 inches. Although, when referring to the above table, you need to also refer to 4.16.3.3 which states that the “size of that portion of the suction pipe located within 10 pipe diameters upstream of the pump suction flange shall be not less than specified in Section 3.28”.

Therefor, you are required to have 80 inches of 8-inch pipe leading to the suction side of the pump. Prior to the 80 inches, you are allowed to have a smaller pipe diameter.

Best of luck with your design,
Alex

Casey Milhorn

9/7/2021 09:14:17 am

Yes, what Jay said. BUT, make sure it works hydraulically in your calculations as well. The pump not meeting 150% is not grounds for failure at commissioning or future fire pump testing (to my understanding).

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Is NFPA 20 Suction the Minimum for Service Size?

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